Gantz follows a very random system. So far, there isn't any pattern on the selected hunters (apart from all of them dying of unnatural causes), so I suppose most of Gantz's actions are quite random, i.e. order of sending. The only exception is when you ask Gantz to go first.

Either mostly-random or, more likely, according to Gantz's personal twisted sense of humor. Note that there's enough time between Takeshi being sent and the next person being sent for him to huddle in fear for a little while, then run in terror far enough that he's out of both sight and hearing.

IMO Gantz is screwing with the kid, the way he might have been in the second hunt when Kurono pops in in front of the oncoming truck.